The proposed study will attempt to establish a reliable, non-invasive method to determine the oxygen saturation of blood in retinal vessels. A spectophotometric technique will be used involving measurements of the optical density of retinal vessels at three different wavelengths. A photoelectric instrument will be constructed for that purpose. Optical properties of blood and the ocular fundus will be investigated, insofar as it is relevant to the proposed method. Trichromatic oximetry of retinal vessels will be used to study retinal oxygenation of normal subjects. Furthermore an investigation on patients with diabetic retinopathy will be undertaken.